With development of integrated circuit technology, application of current sources has become more and more popular. In particular, with wide application of high power white LED lighting and large-sized LED display, a larger number of LEDs are required to be driven, usually in a form of LED strings in series. In order to meet requirements of uniform lighting and white balance, a drive current of each path has to be constant. Therefore, it is particularly crucial to design a drive circuit which can provide substantially uniform constant currents to a plurality of LED paths and ensure that an over current or a zero current is not generated during switching of paths.
A conventional multipath current source switching circuit is as shown in FIG. 1 (having three paths for example). The circuit includes: operational amplifiers 101, 102 and 103; MOS switching tubes M1, M2 and M3; sampling resistances R1, R2 and R3; switches S1 . . . S6; and loads 104, 105 and 106.
When it is switched from a third path to a second path, the switches S2, S3 and S6 are switched on, the switches S1, S4 and S5 are switched off, and in-phase input terminals of the operational amplifiers 101, 102 and 103 are respectively coupled to a reference level Vref and ground. At this moment, the second path is switched on and a first path and the third path are switched off, and current flowing through the MOS tube M2 is an output current and can be expressed by an equation 1. Since the operational amplifier 102, the MOS tube M2 and the sampling resistance R2 form a negative feedback loop, current of a branch in which the MOS tube M2 is present is kept constant. When it is switched from the second path to the first path, the switches S1, S4 and S6 are switched on, the switches S2, S3 and S5 are switched off, and the in-phase input terminals of the operational amplifiers 101, 102 and 103 are respectively coupled to the reference level Vref, ground and ground. At this moment, the first path is switched on and the second and the third path are switched off, and output current is controlled by the first path.
Thus, it can be derived that the operating principle of the conventional multipath current source switching circuit is that, when paths of current circuits are switched, an in-phase input terminal of an error amplifier of a path is coupled to Vref, in-phase input terminals of error amplifiers of the remaining paths are, grounded. Since only one path is switched on at the same time, the current circuits are switched by switching off a previous path while switching on a next path.
                              I          2                =                              V            ref                                R            2                                              (                  Equation          ⁢                                          ⁢          1                )            
When the conventional multipath current source switching circuit is switched between paths of circuits, a path has to be switched on simultaneously when another path is switched off. Since unsynchronized control signals can appear in the following situation: when it is switched from the second path to the first path, both of the in-phase input terminals of the operational amplifier 101 and the operational amplifier 102 are coupled to Vref through switches, a current in a load 104 is expressed by an equation 2, and a current in a load 105 is expressed by an equation 3; when both of the in-phase input terminals of the operational amplifier 101 and the operational amplifier 102 are grounded through switches, both of the currents in the load 104 and the load 105 are zero.
                              I          103                =                                            V              REF                                      R              1                                +                                    V              REF                                      R              2                                                          (                  Equation          ⁢                                          ⁢          2                )                                          I          104                =                              V            REF                                R            2                                              (                  Equation          ⁢                                          ⁢          3                )            
Therefore, when the conventional multipath current source switching circuit is switched among paths of circuits, due to delay of a switching control signal for a circuit or other reasons, two paths can be switched on or switched off at the same time such that the current in the load can be too large or be zero, resulting in failure of the circuit.